A kinetic and DRIFTS study of low-temperature carbon monoxide oxidation over Au-TiO2 catalysts

Titania-supported gold catalysts are extremely active for room temperature CO oxidation; however, deactivation is observed over long periods of time under our reaction conditions Impregnated AuTiO₂ is most active after a sequential pretreatment consisting of high temperature reduction at 773 K, calcination at 673 K and low temperature reduction at 473 K (HTR/C/LTR); the activity after either only low temperature reduction or calcination is much lower. A catalyst prepared by coprecipitation had much smaller Au particles than impregnated AuTiO₂ and was active at 273 K after either an HTR/C/LTR or a calcination pretreatment. Deposition of TiO_x overlayers onto an inactive Au powder produced high activity; this argues against an electronic effect in small Au particles as the major factor contributing to the activity of AuTiO₂ catalysts and argues for the formation of active sites at the AuTiO_x interface produced by the mobility of TiO_x species. DRIFTS (diffuse reflectance FTIR) spectra of impregnated AuTiO₂ reveal the presence of weak reversible CO adsorption on the Au surface but not on the TiO₂; however, a band for adsorbed CO is observed on the pure TiO₂. Kinetic studies with a 1.0 wt.-% impregnated AuTiO₂ sample showed a near half-order rate dependence on CO and a near zero-order rate dependence on O₂ between 273 and 313 K with an activation energy near 7 kcal/mol. A two-site model, with CO adsorbing on Au and O₂ adsorbing on TiO₂, is consistent with Langmuir-Hinselwood kinetics for noncompetitive adsorption, fits partial pressure data well and shows consistent enthalpies and entropies of adsorption. The formation of carbonate and car☐ylate species on the titania surface was detected but it appears that these are spectator species. DRIFTS experiments under reaction conditions also show the presence of weak, reversible adsorption of CO₂ (near 2340 cm⁻¹) which may be competing with CO for adsorption sites.     

The oxidation and scrambling of CO with oxygen at room temperature on Au/ZnO

An FTIR and quadrupole mass study of CO adsorption and oxidation with¹⁶O₂ and¹⁸O₂ on Au/ZnO catalysts is presented. The experimental results indicate that: (i) CO is activated by gold in two molecular forms, a linear carbonyl species bonded at terrace Au sites and a carbonyl species bonded to Au peripheral sites; (ii) a band related to CO adsorbed on Au oxidized sites and a scrambling reaction between CO and¹⁸O₂ indicate that oxygen is also activated on gold sites. The oxygen adsorbed on gold is probably strongly basic, as is the oxygen adsorbed on silver and on copper, and it can easily oxidize CO to CO₂.     

The kinetics of CO oxidation by adsorbed oxygen on well‐defined gold particles on TiO2(110)

Very tiny Au particles on TiO₂ show excellent activity and selectivity in a number of oxidation reactions. We have studied the vapor deposition of Au onto a TiO₂(110) surface using XPS, LEIS, LEED and TPD and found that we can prepare Au islands with controlled thicknesses from one to several monolayers. In order to understand at the atomic level the unusual catalytic activity in oxidation reactions of this system, we have studied oxygen adsorption on Au/TiO₂(110) as a function of Au island thickness, and have measured the titration of this adsorbed oxygen with CO gas to yield CO₂, as function of Au island thickness, CO pressure and temperature. A hot filament was used to dose gaseous oxygen atoms. TPD results show higher O₂ desorption temperatures (741 K) from ultrathin gold particles on TiO₂(110) than from thicker particles (545 K). This implies that O_a bonds much more strongly to ultrathin islands of Au. Thus from Brønsted relations, ultrathin gold particles should be able to dissociatively adsorb O₂ more readily than thick gold particles. Our studies of the titration reaction of oxygen adatoms with CO (to produce CO₂) show that this reaction is extremely rapid at room temperature, but its rate is slightly slower for the thinnest Au islands. Thus the association reaction (CO_g + O_a → CO_2,g) gets faster as the oxygen adsorption strength decreases, again as expected from Brønsted relations. For islands of about two atomic layers thickness, the rate increases slowly with temperature, with an apparent activation energy of 11.4 ± 2.8 kJ/mol, and shows a first‐order rate in CO pressure and oxygen coverage, similar to bulk Au(110).     

Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.     

Reactivity and sintering kinetics of Au/TiO2(110) model catalysts: particle size effects

We review here our studies of the reactivity and sintering kinetics of model catalysts consisting of gold nanoparticles dispersed on TiO₂(110). First, the nucleation and growth of vapor-deposited gold on this surface was experimentally examined using x-ray photoelectron spectroscopy and low energy ion scattering. Gold initially grows as two-dimensional islands up to a critical coverage, θ _cr, after which 3D gold nanoparticles grow. The results at different temperatures are fitted well with a kinetic model, which includes various energetic parameters for Au adatom migration. Oxygen was dosed onto the resulting gold nanoparticles using a hot filament technique. The desorption energy of O_a was examined using temperature programmed desorption (TPD). The O_a is bonded ~40% more strongly to smaller (thinner) Au islands. Gaseous CO reacts rapidly with this O_a to make CO₂, probably via adsorbed CO. The reactivity of O_a with CO increases with increasing particle size, as expected based on Br?nsted relations. Propene adsorption leads to TPD peaks for three different molecularly adsorbed states on Au/TiO₂(110), corresponding to propene adsorbed on gold islands, to Ti sites on the substrate, and to the perimeter of gold islands, with adsorption energies of 40, 52 and 73 kJ/mol, respectively. Thermal sintering of the gold nanoparticles was explored using temperature-programmed low-energy ion scattering. These sintering rates for a range of Au loadings at temperatures from 200 to 700 K were well fitted by a theoretical model which takes into consideration the dramatic effect of particle size on metal chemical potential using a modified bond additivity model. When extrapolated to simulate isothermal sintering at 700 K for 1 year, the resulting particle size distribution becomes very narrow. These results question claims that the shape of particle size distributions reveal their sintering mechanisms. They also suggest why the growth of colloidal nanoparticles in liquid solutions can result in very narrow particle size distributions.     

Adsorption and reaction of CO on CuO–CeO2 catalysts prepared by the combustion method

Temperature-programmed techniques were employed to investigate the interaction of CO with CuO–CeO₂ prepared by the urea-nitrates combustion method. These catalysts exhibited high and stable CO oxidation activity at relatively low reaction temperatures (< 150 °C). The CO adsorption capacity and catalytic activity of the catalysts was analogous to the concentration of easily-reduced copper oxide surface species. TPD and TPSR results can be explained by a dual scheme of CO adsorption: (i) on oxidized sites, which get reduced with simultaneous formation of surface CO₂ and (ii) on reduced sites created by the former interaction. 10–20% of adsorbed CO desorbs molecularly in the absence of gas-phase O₂, but reacts totally towards CO₂ in the presence of gas-phase O₂. Inhibition by CO₂ observed under steady-state CO oxidation conditions is due to CO₂ adsorption as found by CO₂-TPD.     

Activation of a Au/TiO2 Catalyst by Loading a Large Amount of Fe–Oxide: Oxidation of CO Enhanced by H2 and H2O

Catalytic activity of a 1 wt% Au/TiO₂ catalyst is markedly improved by loading a large amount of FeO_x, on which the oxidation of CO in excess H₂ is selectively promoted at temperature lower than 60 °C. Oxidation of CO with O₂ on the FeO_x/Au/TiO₂ catalyst is markedly enhanced by H₂, and H₂O moisture also enhances the oxidation of CO but its effect is not so large as the promotion by H₂. We deduced that activation of Au/TiO₂ catalyst by loading FeO_x is not caused by the size effect of Au particles but a new reaction path via hydroxyl carbonyl intermediate is responsible for the superior activity of the FeO_x/Au/TiO₂ catalyst.     

CO adsorption energy on planar Au/TiO2 model catalysts under catalytically relevant conditions

The adsorption of CO on planar Au/TiO₂ model catalysts was studied by polarization-modulation infrared reflection–absorption spectroscopy (PM-IRAS) under catalytically relevant pressure (10–50 mbar) and temperature (30–120 °C) conditions, both in pure CO and in CO/O₂ reaction gas mixtures. The adsorption energy of CO on the Au particles was determined by a quantitative analysis of the temperature dependence of the CO absorption intensity in adsorption isobars. The data reveal considerable effects of the Au particle size when pure CO is used; the initial adsorption energy decreases from 74 kJ mol⁻¹ (2 nm mean Au particle diameter) to 62 kJ mol⁻¹ (4 nm). For CO/O₂ gas mixtures, the initial CO adsorption energy is, irrespective of the Au particle size, constant at 63 kJ mol⁻¹ (i.e., the CO adsorption energy is reduced for smaller Au particles), but this effect vanishes for larger Au particles.     

In situ FT-IR Spectroscopic Studies on the Mechanism of the Catalytic Oxidation of Carbon Monoxide over Supported Cobalt Catalysts

Three types of supported cobalt catalysts (5% as metal Co loading on SiO₂, Al₂O₃ and TiO₂) were prepared by incipient wetness impregnation with aqueous Co(NO₃)₂·6H₂O solution. Then, all catalysts were calcined in air at 400 °C (assigned as 5Co/Si C400, 5Co/Al C400 and 5Co/Ti C400). Their catalytic activities towards the CO oxidation were studied in a continuous flow micro-reactor. Adsorption of carbon monoxide (CO) and the co-adsorption of CO/O₂ over cobalt oxide were further tested under in situ FT-IR. The results showed that both 5Co/Si C400 and 5Co/Al C400 had higher activity than 5Co/Ti C400. The T₅₀ (50% conversion) for both 5Co/Si C400 and 5Co/Al C400 was reached at temperatures as low as ambient temperature. According to the in situ FT-IR analysis, the variation in oxidation of CO was interpreted with different mechanisms, i.e., the reaction between adsorbed CO and lattice oxygen of cobalt oxide, and part of CO₂ formation via carbonates on 5Co/Si C400; both types of carbonates are formed on 5Co/Al C400 to promote the CO oxidation; while both strong adsorption of CO on TiO₂ and CO₂ on cobalt oxide for 5Co/Ti C400 leads to affect the activity.     

Low-temperature preferential oxidation of CO in a hydrogen rich stream (PROX) over Au/TiO2: Thermodynamic study and effect of gold-colloid pH adjustment time on catalytic activity

By simulating CO and H₂ oxidations at thermodynamic equilibrium and studying the catalytic oxidations over Au/TiO₂, preferential oxidation of CO in a H₂ rich stream (PROX) was investigated. During the simulation, at least two cases under different gaseous feeds, H₂/CO/O₂/N₂ = 50/1/0.5/48.5 or 50/1/1/48 (vol.%) were examined under the assumption of an ideal gas and one atmosphere pressure in the reactor. It was found that the addition of 1% O₂ (the latter case) effectively reduced CO concentration to less than 100 ppm in the temperature range between 0 and 90 °C. This range narrowed to between 0 and 50 °C with the addition of 3% H₂O and 15% CO₂ in the feed. The thermodynamic study suggests that 1% CO in a H₂ rich system can be decreased to below 100 ppm within those low temperature ranges, if there is no substantial adsorptions onto the catalyst surface and the reactions rapidly reach equilibrium. During the catalysis reaction study, a well-pH adjusted Au/TiO₂ catalyst was found very active for PROX. CO conversions at the reactor outlet were close to those at equilibrium. Au/TiO₂ used in this work was prepared via deposition-precipitation (DP) method. The influence of gold colloid pH (at 6) adjustment time on gold loading, gold particle size and chloride residue on TiO₂ surface was detected by atomic absorption (AA), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). A pH adjustment time of at least 6 h for the preparation of gold colloids at room temperature was demonstrated to be essential for the high catalytic activity of Au/TiO₂. This was attributed to the smaller gold particle and the less chloride residue on the catalyst surface.     

Oxidation state of oxide supported nanometric gold

We address the structure gap between surface science and catalysis studies of the activity of oxide supported Au clusters. Reviewing the recent literature we find that surface science investigations often deal with highly reduced systems that have anionic Au clusters and oxygen vacancies in the support. The catalysis studies on the other hand consistently report on oxidized samples with traces of cationic Au. Performing density functional theory calculations we show that the effect of oxidation of oxide supported Au clusters, Au₈/MgO, Au₇/TiO₂ and Au₁₀/TiO₂, is a strong increase in the Au/support adhesion energy and a great structural transformation of the clusters. Some of the Au atoms become positively charged (cationic) in the oxidation process as evidenced indirectly by calculated vibrational stretch frequency shifts of adsorbed CO.     

A Fourier-transform infrared study of CO2 and CO2/H2 interactions with caesium-doped copper catalysts

FTIR spectra are reported of CO₂ and CO₂/H₂ on a silica-supported caesium-doped copper catalyst. Adsorption of CO₂ on a “caesium”/silica surface resulted in the formation of CO₂ ⁻ and complexed CO species. Exposure of CO₂ to a caesium-doped reduced copper catalyst produced not only these species but also two forms of adsorbed carboxylate giving bands at 1550, 1510, 1365 and 1345 cm⁻¹. Reaction of carboxylate species with hydrogen at 388 K gave formate species on copper and caesium oxide in addition to methoxy groups associated with caesium oxide. Methoxy species were not detected on undoped copper catalyst suggesting that caesium may be a promoter for the methanol synthesis reaction. Methanol decomposition on a caesium-doped copper catalyst produced a small number of formate species on copper and caesium oxide. Methoxy groups on caesium oxide decomposed to CO and H₂, and subsequent reaction between CO and adsorbed oxygen resulted in carboxylate formation. Methoxy species located at interfacial sites appeared to exhibit unusual adsorption properties.     

DFT study of catalytic activity of an ultrathin TiO2(110) layer covering Au(112): O2 activation,CO oxidation,and replacing Au with Ag

As a model of Au-titania core-shell catalysts, an ultrathin rutile TiO₂(110) layer inversely supported on Au(112) has been examined by DFT. O₂ adsorbs and activates on the pentacoordinate Ti site of rutile(110) as O₂ π* orbitals accept electronic charge from the Au support via the oxide, as was found for Au/TiO₂. O₂ reacts with CO to yield CO₂ and O, which then reacts with CO to yield CO₂ (barrier ~ 0.6 eV). O₂ adsorption weakens with increasing O₂ coverage and oxide thickness. The reactivity of O₂ changes negligibly even if we replace Au with Ag.     

Electrochemical quartz crystal microbalance study on carbon oxides adsorption in the presence of electrosorbed hydrogen on Pd alloys with Pt and Rh

CO₂ and CO adsorption on Pd-Pt and Pd-Rh alloys has been studied by cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). Adsorbed CO₂ inhibits partially hydrogen adsorption on Pt and Rh surface atoms but does not block significantly hydrogen absorption into alloy bulk. In the presence of adsorbed CO both hydrogen adsorption and absorption are strongly suppressed. On electrodes covered with adsorbed CO the oxidation of previously absorbed hydrogen is significantly shifted into higher potentials. The EQCM response in CO₂/CO adsorption experiments is affected by both the effects connected with the changes in mass attached to the resonator and the non-mass effects including changes in metal-solution interactions and variation of solution density and viscosity in the vicinity of the electrode. Differences in the EQCM behavior suggest that the products of CO₂ and CO adsorption on the alloys studied are not totally identical.     

CO and O2 adsorption on model Au-TiO2 systems

Au/TiO₂ catalysts are active for CO oxidation at room temperature and lower. To probe the surfaces of these catalysts, CO and O₂ adsorption and coadsorption on model Au-TiO₂ systems were examined under UHV conditions using TPD, ASE and XPS. No chemisorption of molecular O₂ was detected, as previously reported for clean Au single-crystal surfaces. A low concentration of CO adsorption sites associated with Au was observed; however, no unique interfacial sites could be unambiguously identified on these surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Transient measurements of lattice oxygen in photocatalytic decomposition of formic acid on TiO2

In the absence of gas-phase O₂, formic acid extracted lattice oxygen from TiO₂ during photocatalytic decomposition (PCD) at room temperature. The amount of oxygen extracted was determined by interrupting PCD of a monolayer of formic acid after various reaction times and measuring O₂ uptake in the dark. After surface oxygen was depleted by PCD, oxygen diffused from the bulk to replenish the surface oxygen vacancies. The rate of oxygen diffusion to the surface was determined by measuring O₂ uptake after various dark times. A small fraction of the CO₂ that formed during PCD remained on the reduced sites of the TiO₂ surface, but this CO₂ was displaced by O₂ adsorption at room temperature.     

Gold dispersed on TiO2 and SiO2: adsorption properties and catalytic behavior in hydrogenation reactions

Titania-supported Au catalysts were given both low temperature reduction and high temperature reduction at 473 and 773 K, respectively, and their adsorption and catalytic properties were compared to identically pretreated Pt/TiO₂ catalysts and pure TiO₂ samples as well as Au/SiO₂ catalysts. This was done to determine whether a reaction model proposed for methanol synthesis over metals dispersed on Zn, Sr and Th oxides could also explain the high activities observed in hydrogenation reactions over MSI (Metal-Support Interaction) catalysts such as Pt/TiO₂. This model invokes O vacancies on the oxide support surface, formed by electron transfer from the oxide to the metal across Schottky junctions established at the metal-support interface, as the active sites in this reaction. The similar work functions of Pt and Au should establish similar vacancy concentrations, and O₂ chemisorption indicated their presence. However, these Au catalysts were completely inactive for CO and acetone hydrogenation, and ethylene hydrogenation rates were lower on the supported Au catalysts than on the supports alone. Consequently, this model cannot explain the high rate of the two former reactions over TiO₂-supported Pt although it does not contradict models invoking specialinterfacial sites.     

The kinetics of CO2 hydrogenation on a Rh foil promoted by titania overlayers

Submonolayer deposits of titania on a Rh foil have been found to increase the rate of CO₂ hydrogenation. The primary product, methane, exhibits a maximum rate at a TiO _x coverage of 0.5 ML which is a factor of 15 higher than that over the clean Rh surface. The rate of ethane formation displays a maximum which is 70 times that over the unpromoted Rh foil; however, the selectivity for methane remains in excess of 99%. The apparent activation energy for methane formation and the dependence of the rate on H₂ and CO₂ partial pressure have been determined both for the bare Rh surface and the titania-promoted surface. These rate parameters show very small variations as titania is added to the Rh catalyst. The methanation of CO₂ is proposed to start with the dissociation of CO₂ into CO_(a) and O_(a), and then proceed through steps which are identical to those for the hydrogenation of CO. The increase in the rate of CO₂ hydrogenation in the presence of titania is attributed to an interaction between the adsorbed CO, released by CO₂ dissociation, and Ti³⁺ ions located at the edge of TiO _x islands covering the surface. Differences in the effects of titania promotion on the methanation of CO₂ and CO are discussed in terms of the mechanisms that have been proposed for these two reactions.     

CO Spillover and Oxidation on Pt/TiO2

The adsorption of CO has been measured on a 2.5 wt% Pt/TiO₂ catalyst using TPD. A somewhat surprising observation is that (i) CO₂ is produced, even though oxygen is not dosed into the system, (ii) repeated experiments result in the same amount of CO₂ desorption. The results appear to be due to a combination of factors–(i) is due to spillover of CO from the Pt to the TiO₂ support, while (ii) is due to the diffusion of Ti³⁺ into the bulk of the TiO₂ crystallite, which effectively removes the surface non-stoichiometry which might otherwise be expected.     

Pulse studies of CH4 interaction with NiO/Al2O3 catalysts

Pulse studies of the interaction of CH₄ and NiO/Al₂O₃ catalysts at 500°C indicate that CH₄ adsorption on reduced nickel sites is a key step for CH₄ oxidative conversion. On an oxygen-rich surface, CH₄ conversion is low and the selectivity of CO₂ is higher than that of CO. With the consumption of surface oxygen, CO selectivity increases while the CO₂ selectivity falls. The conversion of CH₄ is small at 500°C when a pulse of CH₄/O₂ (CH₄O₂=21) is introduced to the partially reduced catalyst, indicating that CH₄ and O₂ adsorption are competitive steps and the adsorption of O₂ is more favorable than CH₄ adsorption